Lydia Parnes, director of the FTC’s Bureau of Consumer Protection, told the AP that 20 sites have already been brought down, and letters have been sent to roughly 20 more, warning them they may be in violation of U.S. law.

Though locateacell.com and celltolls.com are no longer in operation, a number of other data brokers have jumped in to pick up where they left off, Marc Rotenberg, the Electronic Privacy Information Center’s executive director, told the AP.

Rotenberg warns of a practice called "pretexting," in which a cell phone company is spoofed by an impersonator pretending to be a customer, in order to obtain data that can then be sold to interested parties.